This study was conducted to improve the thermal efficiency of the furnace-tube boiler to reduce fuel consumption and harmful exhaust emissions. To achieve this, flow analysis of the 3-pass boiler geometry was conducted by applying the SS κ - ω turbulence model to simulate the flow of water and heat generated by combustion. The performances of the boiler were compared through the thermal efficiency of the boiler and the heat transfer rates in the furnace, 2nd pass, and 3rd pass. It was revealed that the change in materials and number of tubes showed a maximum thermal efficiency difference of 5.6%, which did not significantly impact the boiler performance. The most substantial efficiency improvement was observed with fuel changes, achieving the highest efficiency of 55.39% with LFG. This showed a 21.6% efficiency difference compared to the 33.78% thermal efficiency of LNG.